Technically the Developer Preview builds of Android L that Google issued are meant only for, well, developers. But of course a ton of regular users have downloaded them to try out Lollipop, and those users tend to be the same ones that like to use root apps. The updated Android 5.0 preview builds for the Nexus 5 and Nexus 7 2013 issued yesterday broke the existing root functions, so SuperSU developer Chainfire issued a quick fix.
Unfortunately, that fix doesn't come in the form of an updated app - you'll actually have to flash a modified Android kernel to either device to get root working. As Chainfire explains, download the boot.img file for your N5 or Wi-Fi N7 2013, flash it via the usual fastboot tools on your PC, then go into a custom recovery and flash the SuperSU ZIP file. If any of that doesn't make sense to you, you probably shouldn't be messing with this stuff anyway.
It's worth noting that the Android 5.0 updates for both of these devices should be issued sometime in November, at which point a full re-flash via fastboot will probably be necessary to get back to the retail software path. But if you're dead set on using the preview builds and you rely on root-enabled apps, check the link below for files and basic instructions.
Source: Chainfire (Google+)